A comprehensive study, conducted by a consortium of over 80 researchers from 16 nations, has unveiled a disquieting outlook for the Earth's climate. The massive review, spanning 66 million years of geologic records, places current atmospheric carbon dioxide (CO2) concentrations into context with deep time, shedding light on the potential trajectory of the planet's climate.

Published in the journal Science, the study reveals that the last time CO2 levels consistently reached the levels driven by human activity today was a staggering 14 million years ago. This finding contradicts some previous assessments and underscores the long-lasting effects of greenhouse gas emissions on the climate, which may reverberate for thousands of years.

Lead by Bärbel Hönisch, a geochemist at Columbia University's Lamont-Doherty Earth Observatory, the consortium highlighted that the study provides a more robust understanding of the sensitivity of long-term climate to greenhouse gases. The researchers emphasize that adding CO2 to the atmosphere has long been known to raise temperatures, but the comprehensive analysis now offers a clearer picture of the climate's response over extended periods.

The review raises concerns regarding global warming projections. Mainstream estimates suggest that for every doubling of atmospheric CO2, average global temperatures could rise between 1.5 to 4.5°C (2.7 to 8.1°F) over decades to centuries. However, a recent study argues that the current consensus underestimates planetary sensitivity, projecting a higher temperature increase of 3.6 to 6°C per doubling.

Regardless of the exact figures, all estimates warn of the planet veering dangerously close to, or even surpassing, the 2°C (3.6°F) warming limit that scientists deem crucial to avoid. Currently, atmospheric CO2 concentrations have risen from around 280 parts per million (ppm) in the late 1700s to approximately 420 ppm today, representing an alarming 50% increase. If current trends continue, CO2 levels could reach 600 ppm or more by the end of the century, pushing the planet further along the uncertain warming curve.

The consortium's study did not collect new data, but rather meticulously analyzed existing research to assess its reliability and recalibrated it with the latest analytical techniques. Their efforts resulted in a new CO2 versus temperature curve spanning 66 million years, unveiling what they term as "Earth system sensitivity." According to this measure, doubling CO2 in the atmosphere could lead to an astounding temperature increase of 5 to 8°C.

However, the study's authors emphasize that Earth system sensitivity primarily reflects climate changes occurring over hundreds of thousands of years, highlighting the existence of sluggish, cascading effects that persist for millennia. These effects include the melting of polar ice sheets, diminishing the planet's ability to reflect solar energy, changes in terrestrial plant cover, and variations in clouds and atmospheric aerosols, all of which can influence temperatures.

While the study does not provide exact predictions for the year 2100, it emphasizes the implications for current climate policy. The research reinforces existing knowledge and guides climate modelers striving to forecast future scenarios. The consortium's open database, containing all the project's data, will be continually updated to support ongoing research.

The study further solidifies the generally accepted relationship between CO2 and temperature, refining understanding of specific time periods. For instance, the researchers dispelled the notion that the Earth experienced low CO2 concentrations during the ice-free period from 66 to 56 million years ago. By excluding less dependable estimates, they determined that CO2 was actually relatively high, ranging from 600 to 700 ppm, akin to what could be reached by the end of this century.

The study confirmed that approximately 50 million years ago marked the hottest period recorded, with CO2 levels spiking to as high as 1,600 ppm, leading to temperatures up to 12°C higher than present. Subsequently, around 34 million years ago, CO2 declined sufficiently for the Antarctic ice sheet to begin forming.

Beyond highlighting the impact on climate, the study also emphasizes the influence of CO2 variations on ecosystems. The decline in CO2 over time coincided with the evolution of many modern-day plant and animal species, underscoring the interconnectedness of CO2 levels, climate, and biodiversity.

In light of these findings, Gabriel Bowen, a professor at the University of Utah and co-author of the study, emphasizes that regardless of the precise temperature increase, humanity has already propelled the planet into unprecedented conditions. The consortium's work acts as a warning, prompting a reevaluation of the path forward for the planet.

The consortium has transitioned into a more extensive project aiming to analyze CO2 and climate fluctuations over the entire Phanerozoic eon, spanning 540 million years to the present.

Overall, the review underscores the urgent need for action to mitigate greenhouse gas emissions and curb the devastating consequences of climate change.